1. Field of the Invention
The present invention generally relates to end plug mounting structure for writing instrument, such as a ball-point pen, a mechanical pencil and so on, used for mounting an end plug onto a barrel.
2. Description of the Related Art
In conventional end plug mounting structures, the known structures are as follows: A locking hole is configured around a rear portion of a barrel, and an inserting portion is provided on an end plug, which is inserted into a circular cross section of the rear portion of the barrel from rear end thereof, so that the end plug can be mounted onto the barrel by means of inserting the inserting portion from rear end of the barrel and locking a locking protrusion into the locking hole. However, in the above existing end plug mounting structure for writing instrument, if the inserting portion is inserted from rear end of the barrel, the locking protrusion provided around the inserting portion of the end plug is then tightly pressed by inner surface of the barrel. Therefore, the locking protrusion tends to be worn and distorted, and it is of concern that the locking protrusion cannot be safely locked into the locking hole.
To resolve this problem, a structure is proposed, which is provided with a groove, in which a locking protrusion moves from rear end of a barrel, on inner surface of rear portion of the barrel by being separated backward along lengthwise direction from the locking hole (for example, refer to patent literature 1). Moreover, in the mounting structure, height of the locking protrusion of the end plug is provided less than depth of the groove formed on inner surface of rear portion of the barrel. In such the end plug mounting structure for a writing instrument, when the locking protrusion is moving forward in the groove, since height of the locking protrusion is less than depth of the groove, the locking protrusion is never pressed onto inner surface of the groove by a larger force. The locking protrusion is pressed only on the portion between front end of the barrel and the locking hole, which is located in front of lengthwise thereof. Therefore, abrasion or distortion of the locking protrusion is no longer a problem. Thus, the locking protrusion can be safely locked in the locking hole, and the end plug can therefore be securely fixed on the barrel. See, patent literature 1: Japanese Patent Laid-open No. 2004-98434 (Paragraphs 0006, 0012 and FIG. 1).
However, in recent years, there begins to be a need to remove the end plug mounted on the barrel from the barrel again. To utilized resources effectively, ideas are proposed as follows: if the writing instrument is a ball-point pen, a ball-point pen refill can be replaced from rear portion of the barrel without special opening mechanism for a barrel on front end, which is required previously. Further, if the writing instrument is a mechanical pencil, an end plug mounted on rear portion of the barrel will be removed from there, and a replacing pencil lead refill can be replenished from rear portion of the barrel.
Hereinafter, the locking protrusion, by which an end plug is fixed on a pen, is locked in the locking hole to prevent the end plug from moving backward with respect to the barrel through face-face contact of rear end face thereof and rear surface facing front of the locking hole. In addition, in the previous end plug mounting structures, since the groove is configured by being separated from the locking hole to backside along a lengthwise direction, thus, in order to remove the end plug from the barrel, the end plug is required to move backward along the lengthwise direction of the barrel after the locking protrusion is knocked inside of the barrel. However, it is difficult to knock the locking protrusion, which is locked in the locking hole, inside the barrel; therefore, there exists a problem that the end plug is difficult to remove.
In addition, in above previous end plug mounting structures, wherein the groove is configured by being separated from the locking hole to backside along lengthwise direction, if the end plug is mounted and removed repeatedly, it may happen that the locking protrusion cannot be fully knocked inside the barrel so as to move the end plug backward. Moreover, in the case that the locking protrusion is made of a material such as resin, there also exists a problem that rear surface of the locking hole or rear end face of the locking protrusion, which is in face-face contact with the rear surface, is distorted to present a circular shape, and a force locking the locking protrusion into the locking hole is weaken gradually, so that the end plug cannot be securely fixed on the barrel.